The tear-film coats the surface of the eye in a thin film comprised of three distinct layers. The mucosal layer is closest to the corneal surface, this is followed by the aqueous layer which is saline-like in composition, and the lipid layer forms the outermost tear-film layer and helps reduce evaporation from the eye surface. The thickness of the tear-film layer is on the order of 5 microns and is continuous across the corneal surface until sufficient evaporation has occurred to cause the film to break-up. Blinking aids in the replenishment and restoration of this film. Additionally, there are several areas of the precorneal eye anatomy where tears collect in larger volume: the lacrimal lake (which is a triangular region located by the tear duct), and the inferior tear meniscus (which exists along the lower eyelid). The inferior tear meniscus forms a region approximately 200-300 microns wide that extends up from the lower lid and represents a significant reservoir in terms of tear volume.
Accommodation is a process in which the eye adjusts its focal distance to maintain focus on objects of varying distance. Accommodation is a reflex action, but can be consciously manipulated. Accommodation is controlled by contractions of the ciliary muscle. The ciliary muscle encircles the eye's elastic lens and applies a force on the elastic lens during muscle contractions that change the focal point of the elastic lens.
As an individual ages, the effectiveness of the ciliary muscle degrades. Presbyopia is a progressive age-related loss of accommodative or focusing strength of the eye, which results in increased blur at near distances. This loss of accommodative strength with age has been well studied and is relatively consistent and predictable. Presbyopia affects nearly 1.7 billion people worldwide today (110 million in the United States alone) and that number is expected to rise as the world's population ages. Techniques and devices that can help individuals offset the effects of Presbyopia are increasingly in demand.